Powder-reflection sampling method

Two types of samples are in common use for analysis by the I440F and each includes customized software and sample holders calibrated for individual instruments. The sampling methods include use of (a) dry, powdered samples (approximating those in use in common near infrared reflectance analyzers), and (b) wet monolayers which are essentially samples placed on a large scale microscope slide and a cover glass. 

A number of experimental alternatives to traditional IR transmission spectroscopy are suitable for overcoming some of these complicating experimental factors. In the technique of diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) (Hartauer et al. 1992 Neville et al. 1992) the sample is dispersed in a matrix of powdered alkali halide, a procedure which is less likely to lead to polymorphic transformations or loss of solvent than the more aggressive grinding necessary for mull preparation or pressure required to make a pellet (Roston et al. 1993). For these reasons, Threlfall (1995) suggests that DRIFTS should be the method of choice for the initial IR examination of polymorphs. He has also discussed the possible use of attenuated total reflection (ATR) methods in the examination of polymorphs and provided a comparison and discussion of the results obtained on sulphathiazole polymorphs from spectra run on KBr disks, Nujol mulls and ATR. 

To determine the surface chemical composihon, Fourier transform infrared (FT-IR) spectroscopy with an attenuated total reflection (ATR) method was performed (Nicolet Avator 230). The FT-IR spectrum of the powder sample was obtained with an FT-IR spectrometer (Bruker Tensor 27). 

In addition to the methods described above, the diffuse reflection method (Chapter 12), the attenuated total reflection (ATR) method (Chapter 13), and the infrared microscopy methods (Chapter 16) can be used for measuring infrared spectra from solid samples. The ATR method is particularly suitable for rubbery samples, and it is also useful for powder samples as well as films and liquid- and paste-like samples on substrate plates. The diffuse reflection method may be used to obtain infrared spectra from powder samples appropriately mixed with KCl or KBr powder. The infrared microscope method is useful for examining a sample such as a single fiber or a small crystal. 

If the dirt or contaminant spots can be dissolved in a low boiling-point solvent, solvent extraction (removal) may be an effective method for collecting the sample. If the base plate is a plastic material, the solvent must be carefully selected to ensure that none of the polymer support is dissolved. The solution obtained may then be slowly dripped from a syringe onto a thin film, for example, of polyethylene or polytetrafluoroethylene, and after the solvent has evaporated completely, the remaining powder can then be made into a KBr disk. If the solution is dropped directly onto an infrared-transparent window or a metal plate, the spot often tends to spread to form a circle of the sample after evaporation of the solvent, making the sample then not suitable for either the transmission or transmission-reflection measurement method. If the solution is dropped into a small hole (for example, with a diameter of 1 mm and a depth of 3 mm drilled into a metal support) filled with KBr powder, a diffuse-reflection spectrum may be measured from this after complete evaporation of the solvent. 

For paint purposes, to identify a powder (pigments) sample is desired, its diffraction pattern is compared with diagrams of known substances until a match is obtained. This method requires a library of standard films to be available. Alternatively, d values calculated from the diffraction diagram of the unknown substance are compared with the d values of over 5000 entries listed on plain cards, Keysort cards, and IBM cards in the X-ray powder data file (Switzer et al, 1948). An index volume is available with the file. The cataloging scheme (American Society of Testing Materials, 1989) used to classify different cards lists the three most intense refleetions in the upper left comer of each card. The cards are then arranged in sequenee of decreasing d values of the most intense reflections, based on 100 for the most intense reflection observed. 

If used for continuous monitoring applications and if the sample is a powder then the technique of choice is most likely NIR or Raman. NIR works well with diffuse reflectance, and Raman is comparable in its method of measurement. 

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